1. Technical Field
The present disclosure relates generally to a grounding resistance measurement apparatus and a method of operating the same, and more particularly to a grounding resistance measurement apparatus and a method of operating the same for measuring a grounding resistance at an AC power terminal.
2. Description of Related Art
The UL 2231-1 is a standard for personal protection systems for electric vehicle (EV) supply circuits. The general requirements cover devices and systems intended for used in accordance with the National Electrical Code (NEC) to reduce the risk of electric shock to the user from accessible parts, in grounded or isolated circuits for charging electric vehicles.
In addition, the PE protecting wire is used to connect the all exposed conductive parts of the electrical equipment to provide the equal potential connection system for applications of the distribution system. Because the magnitude of the fault current caused by the isolation destruction is absolutely related to the magnitude of the grounding resistance, the good grounding protection can provide the electrical safety. Accordingly, it is important and beneficial for the grounding fault detection and the grounding protection to accurately detect the condition of the grounding resistance at the AC power terminal.
Reference is made to FIG. 1A, FIG. 1B, and FIG. 1C which are circuit diagrams of conventional grounding resistance detection circuits. The optical coupler Op as shown in FIG. 1A or the transistor switch Q1 and the resistors R1, R2 as shown in FIG. 1B are used to detect and measure the grounding resistance at the AC power terminal. More specifically, a square-wave signal with equal duty cycle would be produced at an output terminal Pa when the grounding resistance Rg is short circuit, as shown in FIG. 2A. On the contrary, the duty cycle of the square-wave signal is varied when the grounding resistance Rg is in a high-impedance state (such as MΩ). Therefore, the condition of the grounding resistance Rg can be detected according to the width of duty cycle of the square-wave signal.
In addition, as shown in FIG. 1C, the waveform of a signal, which is not a square-wave signal as shown in FIG. 2B, produced at the output terminal Pa can be acquired to detect whether the grounding resistance Rg is varied or not. However, the conventional grounding resistance detection circuits exist following disadvantages:
1. The grounding resistance Rg cannot be accurately detected. Because the waveform of the signal produced at the output terminal Pa is detected to judge the condition of the grounding resistance Rg at the AC power terminal, the square-wave signal with equal duty cycle can be judged that the grounding resistance Rg is short circuit. However, the condition of the grounding resistance Rg cannot be accurately detected according to the irregular waveform of the signal produced at the output terminal Pa;
2. The small-scale grounding resistance Rg cannot be detected. The conventional grounding resistance detection circuits are usually used to detect the grounding resistance Rg with at least 50KΩ or 200KΩ. Therefore, the resistance detection results would be extremely wrong once the impedance value of the grounding resistance Rg is lower; and
3. The harm of the human body and the damage of the electrical equipment could occur because the leakage current flowing back to the grounding wire is large.
Accordingly, it is desirable to provide a grounding resistance measurement apparatus and a method of operating the same to accurately calculate a grounding resistance between an equipment grounding point and power grounding point under the protections of the human body and the electrical equipment.